Process and apparatus for the direct reduction of iron oxides in an electrothermal fluidized bed and resultant product

ABSTRACT

A method and an apparatus ( 50 ) for producing direct reduced iron ( 37 ) from dry pellets ( 25 ) composed of iron oxide and carbonaceous material. A mixture of pellets ( 25 ) and free coke particles ( 38 ) with weight relation from 3:1 to 5:1 is fed into the top of an electrothermal fluidized bed ( 32 ) that is fluidized by nitrogen. By exposing pellets ( 25 ) in the electrothermal fluidized bed ( 32 ) to temperatures of between approximately 850-1,100° C. for an average period of between approximately 15-60 minutes, the volatiles are removed and the pellets ( 25 ) metallized. Reduced pellets ( 37 ) mixed with free coke ( 38 ) are discharged from the bottom of fluidized bed ( 32 ) and cooled. The reduced iron pellets ( 37 ) are physically separated from any free coke ( 38 ) and the free coke ( 38 ) is recycled back into the fluidized bed ( 32 ).

BACKGROUND

This invention relates to a process for the chemical reduction of ironoxides into metallic iron at temperatures below the melting point ofiron (1,530° C.), commonly known as “direct reduction.” Morespecifically, the present invention is directed to a process for thedirect reduction (“DR”) of iron ores using solid carbon as the reductantin which the heat needed for the reduction reactions is provided byresistively heating the reactant materials while they are in a fluidizedbed formed by a mixture of reactant materials and free carbonaceousparticles. The invention additionally provides a unique direct reducediron (DRI) product as a result of the inventive process.

Iron ores of various types are found in enormous amounts throughout theworld. Most iron ores are oxides of iron, mainly hematite (Fe₂O₃). Tomake metallic iron, the ore is chemically reduced using agents that arein most cases derived from coal or natural gas (methane). The reductionof iron oxide to form metallic iron is endothermic, that is, heat isrequired to achieve the removal of oxygen atoms from the iron oxidemolecule.

The method most commonly used for the industrial production of iron forsteelmaking is the blast furnace. The furnace itself is a tall, verylarge diameter, generally cylindrical steel vessel that is lined withrefractories.

Lump iron ore, and now more commonly pelletized iron ore, is fed withsolid carbon (coke) into the top of the blast furnace. Air is introducednear the base of the furnace to burn a portion of the carbon to generateheat and carbon monoxide. This gas then reduces the iron oxide intometallic iron. The downward movement of the ore against the rising flowof reducing gas makes the blast furnace a highly efficientcountercurrent reactor for the production of metallic iron from ironores. The partial combustion of the coke provides the necessary heat forthe reduction and for the melting of the metallic iron formed by thereduction. As the iron melts, it trickles downward through the unburnedcoke and forms a pool of molten metal which is periodically tapped andtransferred to other vessels where it is either solidified as a highcarbon “pig iron”, or treated in the molten state to remove carbon andto add alloying agents to form various steel products.

Despite the excellent thermodynamic and thermal efficiency of the blastfurnace for making cast iron, it has many disadvantages. Consequently,during the last 50 years the trend in new plants for production of ironfrom iron ores is to use direct reduction (DR) processing. Indeed, theincrease in iron making capacity in recent years has been largely by DRprocessing.

There are a number of reasons for the rapidly increasing use of DRrather than the well established blast furnace method. For one, iron oredeposits often contain substantial amounts of very fine ore that isunsuitable for use directly in the blast furnace. A number of DRprocesses were originally developed to process iron ore fines withoutthe need to first pelletize or agglomerate the ore.

Another important reason for choosing DR in place of the blast furnaceis that the blast furnace requires “coking” coals. These coals, whenheated to remove much of their volatile content, will form a cokematerial that has a reduced tendency to soften and provides the strengthto support the weight of the burden of iron ore and coke that is movingdownward in the furnace. Although there are numerous large coal depositsthroughout the world, very few have the necessary characteristic of highstrength after coking to be useful in the blast furnace. Thus, DR plantsare generally built in areas of the world where coking coals are notavailable and/or there is an abundant supply of natural gas.

Another reason for the popularity of DR is the flexibility of producinga granular or briquetted iron product that can be shipped elsewhere formelting and production of steel, making the full investment for anintegrated steel mill unnecessary.

DR processes are of two basic types depending on whether the reducingagent is gaseous or it is a form of solid carbon. Most operating DRplants use shaft or fluidized bed (FB) furnaces using gaseous reductantsderived from natural gas. These are known generally by their acronymsMIDREX, HYL, etc. Although natural gas itself is not an effectivereductant, it can be converted into hydrogen or mixtures of hydrogen andcarbon monoxide by “reforming”. The reforming of natural gas is wellestablished technology that involves several process reactors andcatalytic conversion, thus entailing significant capital and operatingcosts.

Gaseous reduction systems are also generally operated at elevatedpressure to increase the reduction rate and productivity per unit ofreactor volume so that the vessel size can be held within the limits ofpractical construction. The elevated pressure requires expensivepressure vessels and the solid feed and product handling must be doneusing “lock hoppers.” This adds significantly to the cost of the DRplant.

A basic disadvantage of DRI processes has been the heat it requires. Thereduction reactions are slow at low temperature, and it is necessary tooperate the reduction reactors at temperatures above 650° C. to achievereasonable processing rates. The direct partial combustion of thereducing gases within the DR reactor is a possible means to achieve theheating, but this procedure is difficult to control and can bedangerous. Instead, the ore is preheated in a separate reactor to atemperature above the desired reduction temperature so that the excessheat provides the heat needed to maintain the desired temperature forreduction. Additionally, the reducing gas is generally preheated byindirect heat exchange before introducing it into the reduction reactor.Preheaters add complexity and cost and are not a highly efficient meansto add heat to the reaction system.

For locations that do not have natural gas, DR processes have beendeveloped that use coal as a solid reductant. The predominant processesof this type have used long, horizontal refractory lined rotary kilns asthe process vessel. These processes are known generally by theiracronyms SL/RN, DRC, and ACCAR/OSIL. Coal, iron ore and some limestoneare fed to the kiln which is operated at temperatures between about850-1,050° C. Auxiliary burners fired with various fuels, includingpulverized coal, are generally provided to heat the charge materialsfrom above. The rate of the reduction reactions with relatively coarsesolid carbon and iron ore is not rapid, and the plants using this typeof reducing reactor are limited in capacity to approximately 100,000metric tons per year. The rotary kiln processes are subject to theproblem of the tendency for the formation of accretions on the wall ofthe kiln. Although rotary kiln based DR plants have been built in manyplaces of the world, the combined annual capacity of these is less than2.5 million metric tons, which constitutes less than 5 percent of thetotal DR production capacity.

Another type of coal or solid carbon reductant process developed duringthe past 20 years employs a rotary hearth rather than a rotary kiln.These processes are known generally by their acronyms Inmetco andFASTMET. To improve the reaction kinetics and to avoid the tendency foragglomeration to occur within the reducing reactor, the rotary hearthprocesses have used a feed of pelletized iron ore fines which alsocontain the carbon reductant within the pellets. Various types of finecoal are generally used as the reductant. The intimate contact betweenthe fine particles of iron ore and the fine carbon, in combination witha significantly higher operational temperature (1,205-1,450° C.),provides a relatively rapid rate of reduction compared with the rotarykiln processes. The heating is also done through use of auxiliaryburners positioned in the roof of the rotary hearth furnace. These canbe operated with various fuels. Heat may also be generated by burning aportion of the combustible gases (primarily CO) produced by thereduction reactions and any volatile matter that evolves from the coal.The source of heat within the rotary hearth furnace, as in the rotarykiln, is generated above the charge material, and the rate of heating islargely by radiation from the flames above. To achieve reasonable ratesof heat transfer to the charge to sustain the reduction, it is necessaryto limit the thickness of the layers of pellets on the hearth to onlytwo or three times the maximal pellet diameter (20-40 mm). Highertemperatures (1,250-1,450° C.) are used than in the rotary kilnprocesses and, with the use of pellets containing the reductant, thereduction rate is higher. However, the throughput is limited by therestriction of having only very thin layers of the reacting pellets onthe hearth.

It is worth noting that while most of the world capacity for directlyreduced iron (DRI) is based on processes that employ gaseous reductantsin moving-beds (shaft kilns), such as the MIDREX and HYL processes, themore recently built DR plants have selected fluidized-bed processes,known by their acronyms FIOR/FINMET and CIRCORED. As discussed above,the fluidized-bed processes offer the advantage of processing iron orefines without requiring pelletization. They also offer very efficientcontact between the particles of iron ore and the gaseous reductant. Inaddition, they provide extremely high heat transfer rates and excellenttemperature control. However, the fluidized-bed processes of the priorart have all used gaseous reductants and have the same limitations asshaft processes as relates to the heat requirements and temperaturelimitations. No prior art DR process has used fluidized-bed technologyfor the direct reduction of iron ore with solid reductants.

Accordingly, it is the primary object of the present invention toprovide a process for the direct reduction of iron oxides in afluidized-bed reaction vessel using solid carbon as the reducing agent.

It is also an object of the present invention to provide a process forthe reduction of iron oxides in a fluidized-bed at significantly highertemperatures, and therefore higher reduction rates (and, thus,production rates), than that achieved in the present processes for thefluidized-bed gaseous reduction of iron oxides.

It is a further object of this invention to provide the heat needed tosustain the reduction reaction and maintain the desired reactiontemperature by use of highly thermally efficient electrothermal orcharge-resistance heating.

It is still another object of this invention to increase the productionrates to rates that are significantly higher than those reported for theprior art DR processes that employ solid reductants.

SUMMARY OF THE INVENTION

These objects, as well as others that will become apparent uponreference to the following detailed description and accompanyingdrawings are provided by a direct reduction process that uses anelectrothermal fluidized-bed (EFB) furnace containing a fluidized-bed ofelectrically conductive granular carbon (coke, char, etc.) which servesas a heating element for the electrothermal conversion of electricalenergy into heat. “Self-reducing” pellets (green pellets for producingDRI) are prepared and comprise fine iron oxide (ore) in intimateassociation with fine carbon, which serves as the agent for reduction ofthe iron oxide. Introducing these pellets into the EFB furnace causesthe pellets to be heated to a temperature of 850-1,100° C., which is notonly sufficient for a high rate of reduction reaction to occur, but alsoto cause the reduced pellets to shrink in size due to sintering of themetallic iron formed by the reduction of the oxide. The shrinkageresults in a lower internal porosity of the pellets and acorrespondingly higher pellet density. This creates a migration of thereduced pellets downward through the upper fluidized bed which containsrelatively low density granular carbon, green pellets and partiallyreduced pellets that serve as the heating element. Electrical power isused to cause electrical current flow through the fluidized bed togenerate heat through the I²R conversion. The application of electricalpower is controlled to maintain the temperature within the EFB at alevel that causes the reduction reaction with simultaneous partialsintering within the pellets.

Another aspect of the invention, the fluidized bed portion of thefurnace, is designed so as to include two distinct zones, namely, anupper zone with vertical walls and a generally constant cross-sectionalarea and a lower zone of a reduced cross-sectional area that terminatesin an inverted conical section in which the fluidizing gas distributoris located. This design provides for active fluidization in the top zoneof a particulate mixture having a high concentration of granular carbonand green DRI pellets and a low concentration of reduced DRI pellets. Amixture of reduced DRI pellets with a low content of granular carbon isfluidized in the lower zone. To prevent particle agglomeration anddeposition formation on the interior of the furnace, the furnace isdesigned without any interior horizontal surfaces. More particularly,all internal surfaces are at an angle equal to or greater than the angleof repose for the reduced pellets. The electrodes that enter thefluidized bed to enable current flow therethrough are positioned to bein contact only with the upper fluidized bed zone that is rich ingranular carbon. This minimizes the possibility of heating the iron-richpellets to a temperature above the melting point of metallic iron, whichwould cause uncontrolled agglomeration of iron onto the surface of theelectrodes and other internal surfaces.

Other objects and advantages of the process of this invention willbecome evident from the following description of the process, equipmentand methods of operation.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects will be more readily apparent byreferring to the following detailed description and the appendeddrawings in which:

FIG. 1 is a vertical cross-sectional view of an electrothermal fluidizedbed furnace according to one aspect of the present invention.

FIG. 2 is a schematic diagram of the process for a method of achievingrapid and efficient reduction of iron oxide in an electrothermalfluidized bed furnace according to another aspect of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In keeping with the invention, the reduction of the iron ore pellets isconducted in a fluidized bed comprising a mixture of coarse carbonparticles (cokes such as petroleum or metallurgical coke, coal with lowsulfer content, pure synthetic graphite, etc.) and partially-reducediron ore pellets. Carbon particles provide electrical conductivity forthe fluidized bed and heat generation internally within the fluidizedbed is due to passing the electrical current through fluidized bed.Further, the carbon-rich fluidized bed zone helps to reduce the productgases back from CO₂ to CO and, if water is present, to CO and H₂. Thedimensions of the dry green pellets and carbon particles are chosen toprovide uniform fluidization of both materials with a degree ofreduction of less than 25-35 percent. The apparent specific density ofDRI pellets changes from 2.1-2.25 g/cc, for pellets with Fe₂O₃ contentof approximately 75 percent (and approximately 50 percent of totaliron), to 4.2-5.2 g/cc for reduced pellets with total iron content ofapproximately 80-85 percent and Fe₂O₃ of approximately 2-5 percent. Theshrinkage in volume of DRI pellets is between approximately 25-35percent. While the inventive process has particular utility for thedirect reduction of iron oxide, other metallic oxides, such as nickeloxide (NiO), vanadium oxide (V₂O₅), tungsten oxide (WO), and oxides ofcobalt (Co) and chromium (Cr), etc., may also be reduced by theinventive process.

It has been found that essentially complete reduction of the iron orecontained within the pellets occurs with less than the stoichiometricconsumption of the carbon contained within the pellets. This suggeststhat some of the free carbon in the fluidized bed serves as a reductantas well as providing electrical conductivity. Thus, the amount of carbonadded to the green pellets conceivably could be reduced or possiblyeliminated. This also suggests that the pelletizing steps may beeliminated, and that iron ore that is crushed and sized could bedirectly introduced into the electrothermal fluidized bed (“EFB”)furnace after preheating. The amount of carbon in the reduced pelletscan also be adjusted to suit the needs of the subsequent steelproduction. (See Example 3, infra.)

Turning to FIG. 1, a mixture of green iron ore pellets containing carbonas the reductant and having a bulk density of about 2.1-2.5 g/cc ismixed with granular carbon having a bulk density of 0.75-1.1 g/cc. Thesedo not necessarily have to be pre-mixed before feeding to furnace. Thesematerials could be fed separately through the same or another feed port.The EFB furnace, generally designated 50, comprises a housing vesselhaving an upper fluidized bed section 52 a with vertical walls and agenerally constant cross-sectional area and a lower fluidized bedportion 52 b that has a reduced cross-sectional area as compared to theupper section 52 a. Preferably the cross-sectional area of the uppersection 52 a is from 2 to 5 times greater than the cross-sectional areaof section 52 b. Stated alternatively, for circular cross-sections, thediameter of section 52 a is from 1.5 to 2.5 times greater than thediameter of section 52 b. As illustrated, the lower fluidized bedportion 52 b is conical in cross-section. However, other shapes may alsobe utilized so long as the reduced cross-sectional area with respect tosection 52 a is maintained. For example, section 52 b may also havevertical walls throughout its major portion and terminate in a conicalsection, with a tapered transition joining sections 52 a and 52 b.

A lower fluidizing gas distribution section 54 depends from section 52 bwhich has sloping walls and a cross-sectional area that decreases fromits upper end toward its bottom. These furnace sections are preferablycylindrical, with a circular horizontal cross-section, or oval, with anellipsoidal horizontal cross-section. Also, several such furnaces may beganged together to form an array, thus providing for increased capacity.

The pellets and granular carbon are fed into the EFB furnace 50 throughinlet 60 and falls through furnace freeboard space 62 and into the upperpart of the fluidized-bed of material contained in section 52 a.Fluidizing gas is made to enter the base of EFB furnace 50 into conicalsection 54 through nozzles 66. As the pellets become reduced, thedensity of the pellets increases to a density higher than that which canbe fluidized by the rising gases within section 52 a. These denserpellets move downward into the lower, smaller diameter fluidized bedsection 52 b and into conical or tapered section 54 where the gasvelocity is higher and thus capable of fluidizing the denser partiallyreduced pellets.

Because the coke particles are substantially less dense than either thegreen pellets or the partially reduced pellets, the majority of the cokeparticles remain in the upper section 52 a, whereas the denser,partially reduced pellets are contained mainly in lower section 52 b.However, because of the substantial turbulence created by the highvelocity of the gas entering section 54, there is considerable mixing ofthe material contained within the two zones of fluidization. When theiron ore pellets are fully reduced, the density becomes 4.2-5.2 g/cc,and at this higher density they move to the lower part of conicalsection 54 and are removed through discharge feeder 64.

As seen in profile, the lower portion 54 has a conical appearance.Importantly, the slope of the walls in the lower section must be steeperthan the angle of repose for the reduced pellets (preferably 15-20° fromvertical) so that none collect on the walls of the lower section. Thereduced cross-sectional area of the lower section 54 promotes theseparation and segregation of the free coke from the pellets and there-circulation of the pellets into the upper section 52 a of the furnace50.

Optionally, in the illustrated embodiment the fluidizing gas is fed tothe nozzles through a tubular heat exchanger 68 that encircles the lowersection 54 before connecting to a manifold that includes the inletnozzles 66. In this way, the temperature of the fluidizing gas is raisedbefore it is injected into the furnace, while the temperature of thereduced pellets that pass through the lower section 54 is lowered to atemperature more likely to inhibit the reoxidation and agglomeration ofthe reduced pellets. Optionally, a separate heat exchanger may beprovided below the bottom of section 54 for which the fluidizing gas mayalso be used as the coolant. The spent fluidizing gas and the gaseousproducts of reduction (CO) exit the freeboard space 62 through a flueoutlet 70. The gas can be advantageously used to dry and pre-heat thegreen pellets and/or as a fuel in related steel making steps in anintegrated steel-making facility to, e.g., melt the reduced pellets,resulting in potential cost savings from reducing the need for otherenergy sources.

The electrothermal heating of the furnace is accomplished by locatingone or more vertically-oriented electrodes 56 of a first polarity spacedfrom the walls of upper fluidized bed section 52 a. There can be anumber of ways to locate electrodes. For example, with a cylindricalprocess vessel and using 3-phase AC electrical power, three electrodescould be located spaced away from the inner walls. If one or moreelectrodes 58 of opposite polarity are located along the inner walls ofupper section 52 a, a voltage applied across the electrodes 56 and 58would cause electric current to flow from electrode(s) 58 through thefluidized-bed of material contained in section 52 a. The electrodes 56,58 may be graphite, baked carbon, etc. The presence of substantialcarbon in section 52 a provides sufficient electrical conductivity toallow current to flow through the bed of fluidized solids contained insection 52 a and generate the heat needed for the reduction by directresistive heating of the material in section 52 a. Although theresistive heating occurs essentially entirely within section 52 a, thehigh degree of mixing of the materials in section 52 a and section 52 bprovides effective heating of the denser iron ore particles throughoutthe time of their residence within the EFB furnace such that thereduction reactions continue to completion.

The residence time of the pellets within the reaction zone (or fluidizedbed) is dependent on the amount of material contained within thefluidized bed and the rate of feeding of the pellets to the furnace. Itfollows therefore that for a furnace of a given cross-section, theresidence time can be varied by varying the feed rate and byindependently varying the height of the fluidized bed in section 52 toaccommodate a greater amount of material. This feature illustrates oneof the major differences in the fluidized-bed furnace from the rotaryhearth type that has limited ability to increase residence time exceptby reducing the throughput.

Referring now to the flow sheet of the inventive method and apparatusfor reduction of iron oxide in an EFB furnace shown in FIG. 2, the ironore is first pelletized in the well-known manner. To this end, theprocess includes three feed bins, 10, 12 and 14, that contain the rawmaterials for the process. Feed bin 10 contains iron oxide (iron orefines, iron oxide concentrates, etc.); feed bin 12 contains solidreductant (green petroleum coke, pulverized coal, coke breeze, petroleumcoke, light coal, etc.); and feed bin 14 contains binder (bentonite,organic resin, etc.). The iron oxide and solid reductant are preferablyin the form of particles sized smaller than 100 mesh, and morepreferably smaller than 150 mesh, to insure good contact therebetween.Further, the solid reductant preferably has a low sulfur content (frombetween 0.3 to 1.0 percent (wt.)), a low ash content (less than 2percent (wt.)), and a volatile content (C_(n)H_(m)) of greater than 2percent (wt.). In the pellets, the carbon reductant is present in anamount preferably from 22.5 percent (wt.) to 28 percent (wt.) of theiron oxide, with the amount of carbon in the pellets being from 1.0 to1.25 times the amount theoretically required for the reduction reactionof Fe₂O₃+3C=2FE+3CO. The ratio may change depending upon the type ofmetal oxide and the reaction conditions.

Raw materials from the feed bins 10, 12 and 14 are mixed together inproper proportions in a blender 16. Preferably, the dry mixture containsapproximately 75 percent (by weight) iron oxide, up to approximately 23percent (by weight) solid reductant, and approximately 1 to 2 percent(by weight) binder. The mixture 17 is then processed through a sizereduction mill 18 to reduce the components to a generallyuniformly-sized powder. The powdered materials are then introduced to apelletizer 20, where water is also added, to form “wet” green pellets21.

The green pellets 21 are transported to a dryer 22 where they are driedat between approximately 110-130° C. (230-260° F.) to remove moisture toless than 0.5-1.0 percent. The exhaust gases from the dryer 22 arepassed through a cyclone 23. Any dust in the gas is recycled back to theblender 16 or to the pelletizer 20 for reprocessing into green pellets.The exhaust gas is then pumped by exhauster 24 through an electrostaticprecipitator or other type dust collector before being exhausted toatmosphere.

The dry green pellets 25 are then sent to screener 26 to separate outpellets that are sized between 6 mesh and 40 mesh (−3.5+0.425 mm). Thepellets larger than 6 mesh are sent to a size reduction mill 27 to bereduced in size, and then returned to the screener 26 to be redivided.Pellets smaller than 40 mesh are recycled directly to the pelletizer 20to be reformed.

The dry green pellets with sizes between 6 and 40 mesh are loaded intothe bin 29, and from it into an EFB furnace 32. As illustrated, the drypellets are passed through a heater 34 before entry into the EFB furnace32. The fresh coke (coal, coke or other material having good electricalconductivity) can be used. This fresh coke can be supplemented with cokerecovered as the non-magnetic fraction by magnet separation of themixture of coke and reduced pellets, is loaded into the EFB furnace fromthe bin 30 in proper proportion with the dry green iron oxide pellets,preferably in a ratio of between about 3:1 and 5:1 (by weight) of ironoxide pellets to free coke. Any low cost, high content granular carbonmaterial with low sulfur content may be used. Preferably, the granularcarbon has a particle size smaller than 3.36 mm (−4 mesh) and largerthan 0.3 mm (+50 mesh). However, the size range may vary depending uponthe green pellet sizes and the velocity of the fluidizing gas.

The solid particles are fluidized by a gas, which may be nitrogen orcarbon monoxide, recirculated furnace gases (such as CO), hydrogen (H₂),and natural gas (CH₄). Importantly, the presence of gaseous reductant inthe fluidizing gas increases the rate of DRI pellet reduction andaffects the residue content of free carbon in the reduced DRI pellets sothat it falls within a range of from 1 to 20 wt. percent. Thus, theinitial carbon content of the green DRI pellets can be reduced to belowthe stoichiometric amount due to the gaseous reductant in the fluidizinggas. The gaseous reductant results from the Bouduar reaction (C+CO₂=2CO)that occurs concurrently with the iron oxide reduction(Fe₂O₃+1.5C=2Fe+1.5 CO₂), and proceeds at a high rate due to thetemperature of 850-1,100° C. and the presence of excess free carbon inthe fluidized bed.

The fluidizing gas is introduced into the bottom of the conical sectionof EFB furnace 32. The green pellets are reduced in the fluidized bed attemperatures between approximately 850-1,100° C. (1,562-2,012° F.) andresidence times of between approximately 15-60 minutes. The reduced DRIpellets 37 and minor amounts of carbon particles 38 are discharged fromEFB furnace 32, cooled in a heat exchanger 36, and sent to a magneticseparator 39. The reduced pellets are preferably cooled to prevent ironreoxidation and particle agglomeration. The reduced DRI pellets with atotal iron content of 85-95 percent and a carbon content of 5-15 percentare separated from the free coke by common physical separation methods,such as magnetic separation.

After separation from the reduced pellets, the free carbon is returnedto the bin 30 and loaded back into the EFB furnace 32 for fluidized bedstabilization. The flue gas 40 from the EFB furnace may be cleaned in acyclone 42. The collected dust is sent back to the bin 30. If a largeamount of iron ore is in the dust, the dust can be stockpiled and thefine particles of iron ore separated out. The flue gas from the cyclone42 is cleaned by a bag house or electrostatic precipitator 44 and may beeither burned and exhausted to the atmosphere or recycled. As shown,part of the clean mixture of nitrogen and carbon monoxide 43 is recycledand burned for pellet drying in the dryer 22, while part of the EFBfurnace flue gas 40 can be used directly, without cleaning, forpreheating of the pelletized iron ore in the heater 34 beforeintroduction into the EFB furnace 32. The hot exhaust from the heater 34can also be recycled to the pellet dryer 22.

As discussed above, an EFB furnace is provided that is particularlysuited for use in the direct reduction of iron oxide set forth above.The fluidized bed of an EFB furnace for DRI pellet production isdesigned to have a fluidized bed zone with a top zone having verticalwalls and constant cross-sectional area and bottom zone having a reducedcross-sectional area, with a smooth transition from the top zone offluidized bed to the bottom zone of the fluidized bed. This provides adecreasing gas velocity from the bottom of the fluidized bed to the topof the fluidized bed. Due to the differences in density of granularcarbon particles and reduced pellets, the granular carbon circulateswithin the entire fluidized bed. DRI pellets with a low degree ofreduction circulate in the top zone of the fluidized bed, while theiron-rich pellets concentrate in the bottom zone of the fluidized bed.Consequently, active mixing of particles occurs within both fluidizedbed zones and rapid heat transfer between the zones is promoted.

EXAMPLE 1 DRI

The production of DRI pellets was conducted at the following operationparameters in a pilot EFB furnace having an inside FB diameter of 61 cm(24 in.). (In this example, as well as Examples 2 and 3, infra., Flexicoke brand of petroleum coke and Desulco 9010 brand of granular carbonwere used for expediency.)

“Green” DRI Pellets Composition: Fine iron oxide (99.3% Fe₂O₃, 100% <100mesh,   −75% <150 μm) Flexi coke (˜6% of volatile components, −23.5%100% <100 mesh) Bentonite (100% <100 mesh)  −1.5% Pellet size: −8 + 40mesh −2.35 + 0.425 mm Specific density: 2.2-2.5 g/cc

Feeding: Mixture of “Green” DRI pellets and heat treated coke “Desulco9010” Composition: “Green” DRI pellets:Desulco 9010 70:30 “Desulco 9010”particle sizes: −4 + 30 mesh 4.75 + 0.6 mm

Process Parameters Feeding rate: 105-115 lb/hr Average power applied120-125 kW Nitrogen rate: 38-45 scf/min Inlet N₂ pressure 30-45 in. H₂OOperation FB temperature: 1020-1050° C. Average retention time of DRIpellets in FB: ˜1 hr Discharge rate of mixture “DRI” pellets and 66-70lb/hr Desulco 9010: Discharge composition: Magnetic fraction (reducedDRI pellets): 70-72% Desulco 9010: 28-30%

Magnetic Fraction Composition: Total Iron (Fe tot): 83-88%, w Femetallic: 76-83%, w C - all carbon chemically free 12-17%, w, Degree ofiron reduction: 92-94% Density of reduced DRI pellets: 4.75-5.2 g/cc

The DRI pellets resulting from this method exhibited a very low rate ofreoxidation. Specifically, the content of metallic Fe in the pellets wassubstantially unchanged when measured six months after reduction. Thisis believed to result from some of the carbon acting as a protectionagainst oxidation, as well as the slow cooling of the reduced pellets ina non-reactive N₂ atmosphere.

EXAMPLE 2 Direct Reduction of Iron Oxide Fines in Electrothermal FBwithout Preliminary Preparation

The direct reduction of iron oxide fines without any preliminarypreparation (i.e., pelletizing) was conducted at the following operationparameters in a pilot EFB furnace with inside FB diameter of 61 cm (24in.). Fine iron oxide: 99.3% Fe₂O₃, 100% <100 mesh, <150 μm Flexi coke:volatile components −2.7% free carbon  −94% sulfur-0.9%  0.9% 100% <100mesh “Desulco 9010” particle sizes: −4 + 30 mesh 4.75 + 0.6 mm

Feeding: Mixture of Iron oxide fines + Flexi coke + “Desulco 9010”Composition: Iron oxide fines 50%, w Flexi coke 15%, w “Desulco 9010”35%, w

Process Parameters: Feeding rate: 85-90 lb/hr Average applied power:95-105 kW Nitrogen rate: 18-25 scf/min Inlet N₂ pressure: 20-35 in. H2OOperation FB temperature: 920-960° C. Average retention time of DRIpellets in FB: ˜1 hr Discharge rate of mixture “DRI” pellets and 48-50lb/hr Desulco 9010:

Discharge Composition: Magnetic fraction (reduced DRI pellets): 40-42%Desulco 9010: 58-60%

Magnetic Fraction Composition: Total Iron (Fe tot):  95-96%, w Femetallic:  91-93%, w C - all carbon chemically free. 1.4-1.5%, w, Degreeof iron reduction:  96-97.6%

EXAMPLE 3 Production of Iron-Bearing Carbon Pellets

In this example, production of iron-bearing carbon (IBC) pellets wasconducted at the following operational parameters in a pilot EFB furnacewith an inside FB diameter of 61 cm (24 in.).

IBC pellets

Composition: Fine iron oxide (99.3% Fe₂O₃, 100% <100 mesh,  50-60% <150μm): Flexi coke (˜6% of volatile components, −38-48% 100% <100 mesh):Bentonite (100% <100 mesh) −2% Pellet size: −8 + 40 mesh −2.35 + 0.425mm Specific bulk density: 1.05-1.55 g/ccFeeding:

Mixture of “Green” IBC pellets and heat treated coke Desulco9010”Composition: “Green” IBC pellets (50/48): Desulco 9010 75:25“Green” IBC pellets (60/38): Desulco 9010 70:30 “Desulco 9010” particlesizes: −4 + 30 mesh 4.75 + 0.6 mm

Process Parameters: Feeding rate: 120-130 lb/hr Average applied power:78-85 kW Nitrogen rate: “Green” IBC pellets (50/48): Desulco 9010 20-21scf/min “Green” IBC pellets (60/38): Desulco 9010 24-25 scf/min Inlet N₂pressure: 20-30 in. H2O Operation FB temperature: 915-950° C. Averageretention time of DRI pellets in FB: ˜45 min Discharge rate of mixtureIBC pellets (50/48) and Desulco 9010: 92-100 lb/hr IBC pellets (60/38):Desulco 9010 105-113 lb/hr Content of magnetic fraction in the dischargeIBC pellets (50/48): ˜59% IBC pellets (60/38): ˜66%

Magnetic Fraction Composition: IBC pellets (50/48): Total Iron (Fe tot):55-59%, w Fe metallic: 49-54%, w C - (all carbon is chemically free)41-45%, w, Degree of iron reduction: 89-92% IBC pellets (60/38): TotalIron (Fe tot): 59-60%, w Fe metal: 57-58.5%, w C - (all carbon ischemically free) 40-41%, w, Degree of iron reduction: 96-98%

Accordingly, a DR process meeting all the objects of the presentinvention has been provided. The EFB furnace provides for a controlledthermal reaction and the reduced cross-section of the fluidizing gasdistribution portion of the furnace promotes segregation of the freecarbon from the reduced pellets. Test results with this process showthat properly formed dry pellets mixed with free coke with a weightrelation from approximately 3:1 to approximately 5:1 can be reduced inEFB furnace with the fluidized bed formed initially by the same type ofcoke at temperatures between approximately 850-1,100° C. with directfurnace heating by electrical power without agglomeration of reducedpellets. The reduction of relatively small sized iron oxide pellets andthe relatively high thermal conductivity and rate of diffusion leads toincreased production by a factor of 2-4 times at temperatures 250-400°C. less than processes using solid carbon reductants. The use ofelectrical power for direct heating provides for a simple furnacedesign. The effluent gases, mainly CO, can be used as a fuel for dryingand pre-heating the iron oxide pellets and for melting the DRI productto achieve improved overall thermal efficiency.

1. A method for the chemical reduction of metallic oxides comprising:forming pellets of metallic oxide and particulate carbon; providing anelectrothermal fluidized bed furnace and establishing a fluidized bed ofgranular carbon therein; heating the fluidized bed of granular carbon bypassing electrical current through said fluidized bed; introducing thepellets of metallic oxide and particulate carbon into the heatedfluidized bed of granular carbon; providing a fluidizing gas through thefurnace at a flow rate sufficient to maintain the granular carbon andmetallic oxide/particulate carbon pellets in the fluidized bed;maintaining the fluidized bed at a temperature sufficient to cause achemical reduction reaction of the metallic oxide and particulate carbonwithin the pellets; removing the chemically reduced pellets from thefurnace; and exhausting effluent gases comprising fluidizing gas andgases resulting from the reduction reaction from the furnace.
 2. Themethod of claim 1 wherein the metallic oxide comprises metallic oxidesthat can be carbothermically reduced to a metallic phase in the presenceof carbon below the melting point of the metal constituent of themetallic oxide.
 3. The method of claim 2 wherein the metallic oxides areselected from the group comprising iron ore, iron oxide, vanadium oxide,nickel oxide, tungsten oxide, cobalt oxide, and chromium oxide.
 4. Themethod of claim 1 wherein the fluidized bed is maintained at atemperature from 850° C. to 1,100° C.
 5. The method of claim 1 whereinthe metallic oxide/particulate carbon pellets are maintained in thefluidized bed for from 15 to minutes to 60 minutes.
 6. The method ofclaim 1 further comprising separating the reduced pellets from anygranular carbon removed from the furnace therewith.
 7. The method ofclaim 6 further comprising recycling the separated granular carbon byreintroducing it into the fluidized bed.
 8. The method of claim 1wherein the fluidizing gas is selected from the group comprisingnitrogen, carbon monoxide, hydrogen and natural gas.
 9. The method ofclaim 1 wherein the effluent gases are recycled to serve as thefluidizing gas.
 10. The method of claim 1 wherein the granular carbon ofthe fluidized bed is selected from the group comprising metallurgicalcoke, petroleum coke, coal and graphite.
 11. The method of claim 1wherein the granular carbon is sized from 0.3 mm (+50 mesh) to 3.36 mm(−4 mesh).
 12. The method of claim 1 wherein the metallic oxide andparticulate carbon both have a particle size of less than 150 μm (−100mesh).
 13. The method of claim 1 wherein the metallic oxide andparticulate carbon both have particle sizes of less than 100 μm (−150mesh).
 14. The method of claim 1 wherein the particulate carbon in thepellets is from 22.5 wt. % to 28 wt. % of the metallic oxide.
 15. Themethod of claim 1 wherein the pellets are sized from 0.425 mm (+40 mesh)to 3.5 mm (−6 mesh).
 16. The method of claim 1 wherein the pressurewithin the fluidized bed is approximately equal to atmospheric pressure.17. A free-flowing directly reduced granular iron pellet containingmetallic iron dispersed within a matrix of partially reduced iron oxidesand free carbon having a particle density within the range of 4.2 g/ccand 5.2 g/cc.
 18. A method for the chemical reduction of metallic oxidescomprising: providing an electrothermal fluidized bed furnace andestablishing a fluidized bed of granular carbon therein; heating thefluidized bed of granular carbon by passing electrical current throughsaid fluidized bed; introducing fine particles of metallic oxide intothe heated fluidized bed of granular carbon; providing a fluidizing gasthrough the furnace at a flow rate sufficient to maintain the granularcarbon and metallic oxide in the fluidized bed; maintaining thefluidized bed at a temperature sufficient to cause a chemical reductionreaction of the metallic oxide and particulate carbon; removing thechemically reduced metallic oxide from the furnace; and exhaustingeffluent gases comprising fluidizing gas and gases resulting from thereduction reaction from the furnace.
 19. The method of claim 18 whereinthe metallic oxide comprises metallic oxides that can becarbothermically reduced to a metallic phase in the presence of carbonbelow the melting point of the metal constituent of the metallic oxide.20. The method of claim 19 wherein the metallic oxides are selected fromthe group comprising iron ore, iron oxide, vanadium oxide, nickel oxide,tungsten oxide, cobalt oxide, and chromium oxide.
 21. The method ofclaim 18 wherein the fluidized bed is maintained at a temperature from850° C. to 1,100° C.
 22. The method of claim 18 wherein the metallicoxide is maintained in the fluidized bed for from 15 minutes to 60minutes.
 23. The method of claim 18 further comprising separating thereduced metallic oxide from any granular carbon removed from the furnacetherewith.
 24. The method of claim 23 further comprising recycling theseparated granular carbon by reintroducing it into the fluidized bed.25. The method of claim 18 wherein the fluidizing gas is selected fromthe group comprising nitrogen, carbon monoxide, hydrogen and naturalgas.
 26. The method of claim 18 wherein the effluent gases are recycledto serve as the fluidizing gas.
 27. The method of claim 18 wherein thegranular carbon of the fluidized bed is selected from the groupcomprising metallurgical coke, petroleum coke, coal and graphite. 28.The method of claim 18 wherein the granular carbon is sized from 0.3 mm(+50 mesh) to 3.36 mm (−4 mesh).
 29. The method of claim 18 wherein themetallic oxide has a particle size of less than 150 μm (−100 mesh). 30.The method of claim 18 wherein the metallic oxide has a particle size ofless than 100 μm (−150 mesh).
 31. The method of claim 18 wherein thepressure within the fluidized bed is approximately equal to atmosphericpressure.
 32. The method of claim 18 further comprising forming pelletsof the fine particles of metallic oxide with particulate carbon prior tointroduction into the heated fluidized bed.
 33. The method of claim 32wherein the particulate carbon in the pellets is from 22.5 wt. % to 28wt. % of the metallic oxide.
 34. The method of claim 32 wherein thepellets are sized from 0.425 mm (+40 mesh) to 3.5 mm (−6 mesh).
 35. Anelectrothermal fluidized bed furnace comprising: a furnace body defininga fluidized bed zone, an overbed zone disposed above the fluidized bedzone, and a discharge zone disposed below the fluidized bed zone, thefluidized bed zone comprising a first portion and a second portiondisposed above the first portion and having a cross-sectional arealarger than that of the first portion, the first portion defining alower fluidizing zone and the second portion defining an upperfluidizing zone; at least one electrode disposed generally centrallywithin the furnace body and extending into the upper fluidizing zone butnot into the lower fluidizing zone; at least one electrode secured tothe wall of the second portion; and a plurality of nozzles disposed atthe bottom of the first portion for introducing fluidizing gas into thefurnace.
 36. The electrothermal fluidized bed furnace of claim 35wherein the first section comprises a conical section defining a centralangle of from 30° to 90°.
 37. The electrothermal fluidized bed furnaceof claim 35 wherein the first section comprises a conical sectiondefining a central angle of from 40° to 60°.
 38. The electrothermalfluidized bed furnace of claim 35 wherein the cross-sectional area ofthe second portion is from 1.5 to 2.5 times larger than thecross-sectional area of the first portion.
 39. The electrothermalfluidized bed furnace of claim 35 wherein the first and second portionshave circular cross-sections and the diameter of the second portion isfrom 1.5 to 2.5 times larger than the diameter of the first portion.